Floating covers are a very economical method of protecting precious water. The cost can be 20% of that for other cover options. Industry-recognized guidelines exist for the design, installation and maintenance of floating cover systems. An example is the American Waterworks Association (AWWA) standard D130-87, which contains the most current standard practices observed for system compliance to US national health and safety regulations. The California-Nevada Section American Water Works Reservoir Floating Cover Guidelines dated March 1999 offers an excellent source of design, operations and maintenance guidelines.
Properly designed reservoir-floating cover systems prevent fluid loss due to evaporation, reduce chemical demand and improve water quality by preventing contamination from bird droppings, airborne particulates, dead animals, pollen and other pollutants. Floating covers block off sunlight preventing algae bloom. They also reduce the production of trialomethane (THM) type compounds such as chloroform from forming that result from the combining of organic substances with chlorine due to reductions in chlorine demand.
Floating cover systems were introduced over 30 years ago. Many have provided a service life beyond 20 years. When first introduced, materials and designs were not developed and in some cases had limited success. Today, with advancements in design and materials, floating covers offer the low cost quality solution of choice where water quality standards require potable water reservoirs be covered.
Floating cover applications range from anaerobic digestion covers for wastewater systems, to potable water reservoir covers for municipal drinking water applications. In farming applications they have been successfully used with enzymes to capture methane gas that is used to fuel electricity producing generators that can satisfy 150% of a typical swine farm's electrical power requirements. This alternative generates renewable “green energy” from an otherwise polluting system that provides zero return on investment.
Floating cover systems offer the best performance when they are constructed with a polyester fabric reinforced “geomembrane” such as HYPALON® or polypropylene. Reinforced cover membranes with United States National Sanitation Foundation Standard No. 61 (NSF-61) approval for contact with drinking water are often encountered in thicknesses of 0.91 mm to 1.14 mm, and are engineered and manufactured to survive the effects of weathering for 20-30 years.
The superior strength of high performance reinforced “geomembranes” enables safe and easy access onto the cover for inspection and repair personnel. The geomembrane is designed to float on liquids or semi-liquid substances and will usually perform one or more of the following functions:                Thermal insulation.        Ultraviolet light insulation.        Encapsulation to create an anaerobic (oxygen less) environment.        Gas containment        Odor control        
The overall physical demands of a floating cover are typically greater than any other application of a geomembrane due to the dynamic forces generated by wind uplift, gas buoyancy etc. Seams are usually very frequent (typically being prefabricated) and are particularly vulnerable to the imposed tensile stresses. Furthermore the encapsulated contaminants often generate extreme chemical conditions.
Floating cover systems require site specific planning and design to be successful. Floating covers must be designed to rise and fall with varying reservoir water levels and have drainage systems for the removal of storm water. Depending upon site, storm water can be conducted through a reservoir and drained to the outside by way of gravity flow or removed from the cover via electrical pumps and discharge hoses. Reservoir access is maintained by the installation of hatches.
There are essentially two types of water-tight floating cover systems commonly specified for drinking water storage applications: a weight tensed defined sump style (such as the commercially available BURKE® style) and a mechanically tensed style (such as the commercially available REVOC® style). Weight tensed defined sump covers use a combination of floats and weights to affect tension and surface geometry. The geometry is controlled in such a manner that strategically located drainage channels or sumps are formed for the collection of storm water. Storm water is conducted through the sump channels to either gravity drains or sump pumps for removal. Mechanically tensed (e.g., REVOC®) type systems use spring or weight loaded cable tension to achieve definition and stability in the cover. As seen in FIG. 1, a REVOC® style floating cover system consists of tensing elements 1 attached to a cover material 2 at specific intervals all around the reservoir's perimeter. The mechanically tensed portion of the cover is thus held into place and becomes a defined plate that is protected from wind uplift and drifting. The outer cover perimeter is relaxed and forms a sump where storm water can be diverted off of the cover through a drainage system. The cover geomembrane material makes direct contact with the top of the water without additional floats for buoyancy. The REVOC® cover system requires less maintenance and lower cover membrane replacement costs and is generally considered a superior system in cold weather applications. Both types of floating covers may be watertight and designed to function at all water levels.
Properly installed floating covers of open water reservoirs should exert minimal stress forces on the cover material to maintain definition and stability. However, floating cover systems should be designed with sufficient slack to accommodate fluctuations in the level of reservoir contents. Some existing methods of floating cover systems (e.g., the REVOC® type systems) consist of tensing elements attached to the cover's interior at specific intervals all around the reservoir's perimeter. These systems use spring or weight loaded cable tension to achieve definition and stability in the cover. As the water levels fluctuate between the minimum and maximum levels, these purely mechanical tensioners have some major weaknesses including very limited cable length and inducing unequal and unnecessary stresses on the cover material.